There are many medical situations where it is necessary or desirable to implant a stent within a patient in order to prevent or counteract a constriction in a naturally occurring vessel or passage. In this context, a “stent” is an artificial tubular structure which is able to apply force radially outwardly on a vessel or passage of a patient in order to maintain patency of the vessel or passage and permit fluid flow through the vessel or passage.
The most common procedure in which a stent is implanted in a patient is implantation in a coronary artery which has become partially blocked or occluded (referred to as being “stenosed”) by a lesion or plaque. In this procedure, a stenosed coronary artery is opened through an angioplasty procedure in which a crimped stent is introduced into the stenosed artery and the stent is expanded within the artery, for example, by using a balloon on a catheter. Expansion of the stent compresses the lesion or plaque blocking the coronary artery and allows blood to flow through the artery without constriction. As part of the procedure, the stent is left in place in the artery, in expanded form, in order to maintain the patency of the artery. In some procedures, prior to implantation of the stent, a pre-dilation step is carried out by expanding a balloon on a catheter within the section of the coronary artery affected by the lesion in order to compress the lesion or plaque prior to insertion of the stent. Vascular stents are also used in other blood vessels aside from coronary arteries and the implantation procedure is similar.
One of the main requirements of a vascular stent is that it can be enlarged from a crimped configuration which has a sufficiently small radial diameter in order to be guided in an angioplasty procedure, to an expanded configuration in which the exterior surface of the stent contacts and engages with the inner surface of the blood vessel. Moreover, in the expanded configuration, the stent must have sufficient radial strength in order to maintain the lumen of the blood vessel open. There are various different forms of construction of vascular stents but a common form is a metal mesh stent in which the stent comprises a network of struts which delineate a plurality of cells within the network. The struts are hinged or otherwise deformable with respect to each other which permits expansion of the stent after implantation. However, more recently, mesh stents have been made from other materials such as biodegradable polymers.
One problem with mesh stents is that of “arterial recoil”. After a mesh stent has been expanded, the stent may not have sufficient radial strength to withstand the radially inward force of the blood vessel such that the stent is squeezed and the blood vessel constricts. There are various solutions to this problem, although none is ideal. For example, one solution is to reduce the cell size of the mesh which directly increases the radial strength of the stent. However, the problem with this approach is that decreasing the cell size reduces the flexibility of the stent which can make implantation of the stent difficult because blood vessels are not perfectly cylindrical in shape and thus the natural conformation of a blood vessel may be lost when the stent is implanted.
Another solution is to increase the thickness of the strut size. However, there is evidence that suggests that the thicker the struts of a mesh stent, the greater the likelihood of restenosis after implantation of the stent.
The problem of arterial recoil in mesh stents can occur with metal mesh stents but is particularly a problem with polymer mesh stents which have less intrinsic strength than metal mesh stents.
U.S. Pat. No. 6,059,822 reports on a mesh stent that has large mesh portions at either end of the stent and a small mesh portion at the longitudinal centre of the stent. The small mesh portion has a mesh of a smaller size than the larger mesh portion and is used at the longitudinal centre of the stent where the main lesion is located once the stent is implanted. The small mesh portion provides more radial strength and lessens any chance of prolapse. The ends of the stents, however, have a larger mesh size so as to reduce the damage to healthy tissue parts of the artery in which the stent is located. Thus there are only two different cell sizes in the stent.
However, there is always a demand to improve upon the configuration of mesh stents so as to avoid the problem of arterial recoil along the longitudinal length of the stent whilst maintaining flexibility and minimising arterial injury.
The background to the invention has been explained above in relation to vascular stents but it is to be understood that the present invention is not limited thereto. Stents other than vascular stents exist such as ureteral, urethral, duodenal, colonic and biliary stents, and analogous problems arise with these stents as have been described above in relation to vascular stents.
The present invention seeks to alleviate one or more of the above problems.